Development of digital technologies for recording and playing back images and sound, and for transmitting these in a free space or over a cable, is now reaching a point where it is evident that they offer significant advantages in comparison with conventional analogue techniques. Digital communications can provide great image and sound quality, spectrum and power efficiency, service flexibility, multimedia convergence, and, potentially, lower equipment costs. Use of digitized signals for delivery of video services to individual subscribers is continually growing, and has already become a dominant form of distribution in many parts of the world.
Quadrature amplitude modulation (QAM) is frequently employed to encode a stream of digital data onto an electrical signal. QAM typically uses a pair of sinusoidal radio-frequency waves phase shifted at 90 degrees with respect to each other. Each of the waves is amplitude modulated at a discrete set of amplitudes, including “negative” amplitudes corresponding to the phase shift of 180 degrees relative to the “positive” amplitudes. Each combination of the amplitudes of the two waves represents a transmitted digital symbol. An X-Y orthogonal plot of the amplitudes of the two waves is called a constellation. The more symbols are in a QAM constellation, the more bits per symbol can be transmitted. The symbols are decoded at a receiver location. A single QAM transmission line can include a plurality of transmission channels at individual frequencies of the carrier sinusoidal waves.
Electrical interference and other imperfections and disturbances of a digital transmission line can distort transmitted QAM signals. The electrical interference can create a background noise in one or more transmission channels. The background noise can be relatively constant, or be pulsed in nature.
Due to their intermittent character, pulsed QAM signal impairments are particularly difficult to detect and control. A QAM spectrum analyzer can capture constellation, modulation error ratio (MER), or error vector magnitude (EVM) vs. time. These captures can occur when trigger conditions are met. Triggering is essential in order to capture intermittent impairments, because an instrument capable of capturing sufficiently high resolution for hours at a time would be prohibitively expensive.
QAM diagnostic instruments developed to date can capture high resolution data for a short time. Diagnostic instruments of the prior art can also perform triggered captures. The prior art instruments can monitor the QAM signal and count high-magnitude pulses within a given spectral band, and/or capture high-resolution QAM data once a high-magnitude pulse is detected.
By way of example, Williams in U.S. Pat. No. 6,151,559 discloses a system and a method for characterizing an undesirable noise within a selected frequency band. Referring to FIG. 1, a test system 100 for characterizing the nature and the severity of the impairments affecting a radio frequency signal path is presented. Testing is done by monitoring the output of an unused signal path 102 of a downconverter 103 with a bandpass filter 104 and a totaling counter 106. The bandpass filter 104 passes impairment energy from the signal path 102 to the counter 106 in a frequency band of interest, thereby increasing the count value on the counter 106. The bandpass filter 104 limits the ability of impairments or signals from other frequency bands to increase the count value of the counter 106. The counter's 106 input threshold voltage level is set to trigger on impairments that are sufficiently strong to cause data errors. The count value may be used to determine the time duration of an impairment by dividing the count accumulated in one second by the center frequency of the filter. An optional digital time trace acquisition unit can be used to capture digital time traces of the QAM signal to help identify the source and the nature of the impairment.
While these capabilities are helpful, instruments developed to date lack adequate capabilities of capturing impairment data. Diagnostic instruments of the prior art can capture a QAM signal when the signal level increases above a preset threshold. Unfortunately, the filter 104 reduces the video bandwidth so much that it can also filter out the impulse noise causing the impairment.
Accordingly, it is a goal of the invention to provide a device and a method for capturing and characterizing transient impairments in a QAM digital channel.